Multi-layer spectral purity filter, lithographic apparatus including such a spectral purity filter, device manufacturing method, and device manufactured thereby

ABSTRACT

A multi-layered spectral purity filters improveS the spectral purity of an Extreme Ultra-Violet (EUV) radiation beam and also collect debris emitted from a radiation source.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lithographic apparatus and a methodof using the apparatus in the manufacture of a device such as anintegrated circuit (IC). In particular, the present invention relates tomulti-layered spectral purity filters which improve the spectral purityof an Extreme Ultra-Violet (EUV) radiation beam and also filter debrisemitted from a radiation source.

2. Description of the Related Art

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.including part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude steppers, in which each target portion is irradiated by exposingan entire pattern onto the target portion at one time, and scanners, inwhich each target portion is irradiated by scanning the pattern througha radiation beam in a given direction (the “scanning” direction) whilesynchronously scanning the substrate parallel or anti-parallel to thisdirection. It is also possible to transfer the pattern from thepatterning device to the substrate by imprinting the pattern onto thesubstrate.

In addition to EUV radiation, an EUV source emits many differentwavelengths of radiation and debris. This non-EUV radiation is harmfulfor an EUV lithography system, so it has to be removed by, for example,a spectral purity filter. Present spectral purity filters are based onblazed gratings. However, these gratings are difficult to produce, sincethe surface quality of a triangular shaped pattern on the spectralpurity filters has to be very high. The roughness of the surface shouldbe lower than 1 nm RMS.

Debris mitigation schemes may be applied for suppressing debris emittedfrom radiation sources. However, commonly used debris mitigationmethods, which include foil traps and gas buffers, do not guaranteeeffective debris protection. Moreover, use of standard (e.g. Zr) thinfilters transmissive for EUV is virtually impossible due to thefragility of the filters and low heat-load threshold.

Debris mitigation schemes may also involve physical removal ofcomponents from a lithographic apparatus and their off-line cleaningusing chemical processes. However, having to accommodate such off-linecleaning makes vacuum and mechanical design of a lithographic apparatusextremely complicated. Off-line cleaning also involves a significantamount of down time for the lithographic apparatus.

A further problem of existing spectral purity filters is that theychange the direction of a radiation beam from an EUV source. Therefore,if a spectral purity filter is removed from an EUV lithographyapparatus, a replacement spectral purity filter has to be added or amirror at a required angle has to introduced. The added mirrorintroduces unwanted losses into the system.

I

SUMMARY OF THE INVENTION

It is an aspect of the present invention to provide a spectral purityfilter which is capable of mitigating debris emitted from a radiationsource and also improving the spectral purity of a radiation beam.

According to an embodiment of the present invention there is provided alithographic spectral purity filter including a multi-layered structureof alternating layers, wherein the spectral purity filter is configuredto enhance the spectral purity of a radiation beam by reflecting orabsorbing undesired radiation, the spectral purity filter also beingconfigured to collect debris emitted from a radiation source.

Undesired radiation may be defined as radiation which has a differentwavelength from desired radiation of the radiation beam, which may forexample be EUV radiation. The undesired radiation, which may bereflected or absorbed, may have wavelengths which are larger or smallerthan the desired radiation of the radiation beam.

The spectral purity filter may suppress undesired radiation whiletransmitting desired radiation with low wavelengths, such as EUVradiation. The multi-layered structure may therefore be designed andadapted to reflect or absorb undesired radiation (e.g. DUV) whiletransmitting desired radiation (e.g. EUV).

The spectral purity filter of the present invention may be classified asa transmissive filter. The spectral purity filter may have atransmission of at least 40%, at least 60%, at least 80% and preferablyat least 90% for desired radiation such as EUV radiation.

The spectral purity filter may filter out undesired radiation, such asDUV radiation. For example, on the transmission of radiation through thespectral purity filter, the ratio of EUV radiation to DUV radiation maybe enhanced by about 100 times, 1000 times or even up to about 10⁵times. A significant improvement in the spectral purity of a radiationbeam on transmission through a spectral purity filter according to thepresent invention may therefore be obtained.

The multi-layered structure of the spectral purity filter may have about2-200 alternating layers, about 10-100 alternating layers, or about20-50 alternating layers. The alternating layers may have a thickness ofabout 0.2 to 100 nm, about 0.2 to 20 nm, or about 0.5 to 5 nm. Each ofthe alternating layers may form continuous layers of substantiallyconstant thickness. The total thickness of the multi-layered structureof alternating layers may range from about 10 to 700 nm and preferablyabout 100 to 200 nm.

The multi-layered structure of alternating layers may be formed from anysuitable number of different alternating layers. For example, there maybe two different layers which alternate with one another. Alternatively,there may be three different layers which alternate with one another.

The alternating layers forming the multi-layered structure may be formedfrom a combination of any of the following: Zr and Si layers; Zr and B₄Clayers; Mo and Si layers; Cr and Sc layers; Mo and C layers; and Nb andSi layers. The spectral purity filter including the multi-layeredstructure of alternating layers may be formed by depositing alternatinglayers of, for example, Zr and Si, using any suitable technique such asmagnetron sputtering, epitaxy, ion sputtering and e-beam evaporationwith or without ion polishing.

The multi-layered structure of the spectral purity filter may bedesigned to be strong and robust so that the filter is not damaged bydebris emitted from a radiation source.

The multi-layered structure of alternating layers may be deposited ontoa mesh-like structure. The mesh-like structure may be in the form of ahoneycomb structure and may penetrate from one side of the multi-layeredstructure to the other. The mesh-like structure may include a pluralityof apertures within which material forming the multi-layered structureof alternating layers may be deposited. The mesh may be formed from anysuitable electroformable material, for example Ni and Cu. The aperturesin the mesh-like structure may have a size range of about 0.01-5 mm²,for example about 1-1.5 mm². The mesh-like structure may improve thestrength of the multi-layered structure in the spectral purity filter.The spectral purity filter may therefore be made of a thinnermulti-layered structure in comparison to spectral purity filters withouta mesh. This may improve the transmission of EUV radiation. A spectralpurity filter including a mesh-like structure may therefore besubstantially strengthened and may withstand greater pressuredifferences in comparison to filters with no mesh-like structure.

Spectral purity filters according to the present invention with a totalthickness of about 50-600 nm including a mesh-like structure penetratingfrom one side of the filter to the other and with apertures of about 1mm², and with a total surface area of about 1 cm² may withstand pressuredifferences of up to about 0.5-1 bar.

A mesh-like structure may be placed adjacent to one side only of thealternating layers of the multi-layered structure. A mesh-like structuremay be placed adjacent both sides of the alternating layers of themulti-layered structure. In these embodiments, the mesh-like structuredoes not penetrate into the alternating layers.

A mesh-like structure may partially penetrate into the alternatinglayers of the multi-layered structure.

There may be no mesh-like structure. Spectral purity filters accordingto the present invention with a total thickness of about 50-600 nm andnot including a mesh-like structure, and with a total surface area ofabout 1 cm² may withstand pressure differences of up to about 0.1 mbar.

The spectral purity filters of the present invention may withstand aheat flux up to about 6 W/cm² and even higher. In addition, the spectralpurity filters may withstand temperatures of up to about 500° C. andeven higher, such as up to 1000° C. to 1500° C. This is much higher thanactually required in a standard lithographic apparatus.

The spectral purity filters according to the present invention may beconnected in a modular form allowing large surface areas of up to about1 to 10 cm² to be formed by a combination of many spectral purityfilters.

The spectral purity filters may be positioned at any point in alithographic apparatus apart from an intermediate focus of a radiationbeam. For example, the spectral purity filters may be positioned in asource-collector-module or in an illumination system of a lithographicapparatus. The spectral purity filter may be positioned downstream of acollector and upstream of an intermediate focus. Where debris mitigationis desirable, the spectral purity filter may be positioned upstream of acollector in a lithographic apparatus. Where the spectral purity filteris to be used mainly for spectral filtering, then the spectral puritymay be positioned downstream of a collector such as at any of thefollowing positions: between an intermediate focus of a radiation beamand an incidence reflector; between an incidence reflector and a masktable; or above a substrate table.

The spectral purity filter according to the present invention is alsocapable of filtering and mitigating debris emitted from a radiationsource. The debris emitted from a radiation source may be atomicparticles, micro-particles and ions. The spectral purity filtersaccording to the present invention may be used in combination with otherdebris suppression devices such as foil traps, background gas pressuresystems, electromagnetic suppressors and any other suitable devices.

The spectral purity filters may also be easily removed from alithographic apparatus, and then cleaned externally and repositioned inthe lithographic apparatus, or substituted with a replacement spectralpurity filter. By easy replacement of the spectral purity filter, thisovercomes the need to disassemble a substantial part of a lithographicapparatus. The spectral purity filter of the present invention thereforehas cost benefits in comparison to spectral purity filters existing inthe prior art.

According to a further embodiment of the present invention there isprovided a lithographic apparatus including: an illumination systemconfigured to condition a radiation beam; a support configured tosupport a patterning device, the patterning device being configured toimpart the radiation beam with a pattern in its cross-section to form apatterned radiation beam; a substrate table configured to hold asubstrate; and a projection system configured to project the patternedradiation beam onto a target portion of the substrate, wherein aspectral purity filter including a multi-layered structure ofalternating layers is configured to enhance the spectral purity of theradiation beam by reflecting or absorbing undesired radiation, thespectral purity filter also being configured to collect debris emittedfrom a radiation source.

The spectral purity filter may be positioned in asource-collector-module or an illumination system of the lithographicapparatus. The spectral purity filter may be positioned downstream of acollector and upstream of an intermediate focus of the radiation beam.

According to a yet further embodiment of the present invention there isprovided a lithographic apparatus including a spectral purity filterincluding a multi-layered structure of alternating layers, wherein thespectral purity filter is configured to enhance the spectral purity of aradiation beam by reflecting or absorbing undesired radiation, thespectral purity filter also being configured to collect debris emittedfrom a radiation source.

According to a further embodiment of the present invention there isprovided a device manufacturing method including providing a conditionedradiation beam using an illumination system; imparting a pattern to theradiation beam; projecting the patterned beam of radiation onto a targetportion of the substrate; wherein a spectral purity filter including amulti-layered structure of alternating layers is configured to enhancethe spectral purity of the radiation beam by reflecting or absorbingundesired radiation, the spectral purity filter also being configured tocollect debris emitted from a radiation source.

According to a further embodiment of the present invention there isprovided a device manufacturing method including projecting a patternedbeam of radiation onto a substrate, wherein a spectral purity filterincluding a multi-layered structure of alternating layers is configuredto enhance the spectral purity of the radiation beam by reflecting orabsorbing undesired radiation, the spectral purity filter also beingconfigured to collect debris emitted from a radiation source.

According to a further embodiment of the present invention there isprovided a device manufactured according to a method described above.

The manufactured device may, for example, be an integrated circuit (IC),an integrated optical system, a guidance and detection pattern formagnetic domain memories, a liquid crystal display (LCDs) and athin-film magnetic head.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying schematic drawings inwhich corresponding reference symbols indicate corresponding parts, andin which:

FIG. 1 schematically depicts a lithographic apparatus according to anembodiment of the present invention;

FIG. 2 schematically depicts a lithographic apparatus according toanother embodiment of the present invention;

FIG. 3 schematically depicts a spectral purity filter according to anembodiment of the present invention;

FIG. 4 schematically depicts a cross-section of part of the spectralpurity filter shown in FIG. 3;

FIG. 5 schematically depicts a spectral purity filter according toanother embodiment of the present invention;

FIG. 6 schematically depicts a cross-section of part of the spectralpurity filter shown in FIG. 5;

FIG. 7 represents calculated and measured transmission values forspectral purity filters according to embodiments of the presentinvention;

FIG. 8 schematically depicts an apparatus in which properties of aspectral purity filter according to an embodiment of the presentinvention are measured;

FIG. 9 schematically depicts another apparatus in which properties of aspectral purity filter according to an embodiment of the presentinvention are measured; and

FIGS. 10 a-10 d depict exposed and unexposed parts of a spectral purityfilter according to an embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus including anillumination system (illuminator) IL configured to condition a radiationbeam B (e.g. UV radiation or EW radiation). A support (e.g. a masktable) MT is configured to support a patterning device (e.g. a mask) MAand is connected to a first positioning device PM configured toaccurately position the patterning device in accordance with certainparameters. A substrate table (e.g. a wafer table) WT is configured tohold a substrate (e.g. a resist-coated wafer) W and is connected to asecond positioning device PW configured to accurately position thesubstrate in accordance with certain parameters. A projection system(e.g. a refractive projection lens system) PS is configured to project apattern radiation beam B onto a target portion C (e.g. including one ormore dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, todirect, shape, and/or control radiation.

The support supports, e.g. bears the weight of, the patterning device.It holds the patterning device in a manner that depends on theorientation of the patterning device, the design of the lithographicapparatus, and other conditions, for example whether or not thepatterning device is held in a vacuum environment. The support can usemechanical, vacuum, electrostatic or other clamping techniques to holdthe patterning device. The support may be a frame or a table, forexample, which may be fixed or movable as required. The support mayensure that the patterning device is at a desired position, for examplewith respect to the projection system. Any use of the terms “reticle” or“mask” herein may be considered synonymous with the more general term“patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located, for example, between theprojection system and the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives radiation from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation is passed from the source SO tothe illuminator IL with the aid of a beam delivery system BD including,for example, suitable directing mirrors and/or a beam expander. In othercases the source may be an integral part of the lithographic apparatus,for example when the source is a mercury lamp. The source SO and theilluminator IL, together with the beam delivery system BD if required,may be referred to as a radiation system.

The illuminator IL may include an adjusting device AD configured toadjust the angular intensity distribution of the radiation beam.Generally, at least the outer and/or inner radial extent (commonlyreferred to as σ-outer and σ-inner, respectively) of the intensitydistribution in a pupil plane of the illuminator can be adjusted. Inaddition, the illuminator IL may include various other components, suchas an integrator IN and a condenser CO. The illuminator may be used tocondition the radiation beam, to have a desired uniformity and intensitydistribution in its cross-section.

The radiation beam B is incident on the patterning device (e.g., maskMA), which is held by the support (e.g., mask table MT), and ispatterned by the patterning device. Having traversed the mask MA, theradiation beam B passes through the projection system PS, which projectsthe beam onto a target portion C of the substrate W. With the aid of thesecond positioning device PW and a position sensor IF (e.g. aninterferometric device, linear encoder or capacitive sensor), thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the radiation beam B.Similarly, the first positioning device PM and another position sensor(which is not explicitly depicted in FIG. 1 but which may be aninterferometric device, linear encoder or capacitive sensor) can be usedto accurately position the mask MA with respect to the path of theradiation beam B, e.g. after mechanical retrieval from a mask library,or during a scan. In general, movement of the mask table MT may berealized with the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which form part of the firstpositioning device PM. Similarly, movement of the substrate table WT maybe realized using a long-stroke module and a short-stroke module, whichform part of the second positioning device PW. In the case of a stepper,as opposed to a scanner, the mask table MT may be connected to ashort-stroke actuator only, or may be fixed. Mask MA and substrate W maybe aligned using mask alignment marks M1, M2 and substrate alignmentmarks P1, P2. Although the substrate alignment marks as illustratedoccupy dedicated target portions, they may be located in spaces betweentarget portions (these are known as scribe-lane alignment marks).Similarly, in situations in which more than one die is provided on themask MA, the mask alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

-   1. In step mode, the mask table MT and the substrate table WT are    kept essentially stationary, while an entire pattern imparted to the    radiation beam is projected onto a target portion C at one time    (i.e. a single static exposure). The substrate table WT is then    shifted in the X and/or Y direction so that a different target    portion C can be exposed. In step mode, the maximum size of the    exposure field limits the size of the target portion C imaged in a    single static exposure.-   2. In scan mode, the mask table MT and the substrate table WT are    scanned synchronously while a pattern imparted to the radiation beam    is projected onto a target portion C (i.e. a single dynamic    exposure). The velocity and direction of the substrate table WT    relative to the mask table MT may be determined by the (de-)    magnification and image reversal characteristics of the projection    system PS. In scan mode, the maximum size of the exposure field    limits the width (in the non-scanning direction) of the target    portion in a single dynamic exposure, whereas the length of the    scanning motion determines the height (in the scanning direction) of    the target portion.-   3. In another mode, the mask table MT is kept essentially stationary    holding a programmable patterning device, and the substrate table WT    is moved or scanned while a pattern imparted to the radiation beam    is projected onto a target portion C. In this mode, generally a    pulsed radiation source is employed and the programmable patterning    device is updated as required after each movement of the substrate    table WT or in between successive radiation pulses during a scan.    This mode of operation can be readily applied to maskless    lithography that utilizes programmable patterning device, such as a    programmable mirror array of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIG. 2 shows a side view of an EUV lithographic apparatus in accordancewith an embodiment of the present invention. It will be noted that,although the arrangement is different to that of the apparatus shown inFIG. 1, the principle of operation is similar. The apparatus includes asource-collector-module or radiation unit 3, an illumination system ILand a projection system PL. Radiation unit 3 is provided with aradiation source LA which may employ a gas or vapor, for example Xe gasor Li vapor, in which a very hot discharge plasma is created so as toemit radiation in the EUV range of the electromagnetic radiationspectrum. The discharge plasma is created by causing a partially ionizedplasma of an electrical discharge to collapse onto the optical axis O.Partial pressures of 0.1 m bar of Xe, Li vapor or any other suitable gasor vapor may be required for efficient generation of the radiation. Theradiation emitted by radiation source LA is passed from the sourcechamber 7 into collector chamber 8 via a gas barrier or foil trap 9. Thegas barrier includes a channel structure. The collector chamber 8includes a radiation collector 10 which is formed, for example, by agrazing incidence collector. Radiation passed by collector 10 transmitsthrough a spectral purity filter 11 according to the present invention.It should be noted that in contrast to blazed spectral purity filters,the spectral purity filter 11 does not change the direction of theradiation beam. The radiation is focused in a virtual source point 12(i.e. an intermediate focus) from an aperture in the collection chamber8. From chamber 8, the beam of radiation 16 is reflected in illuminationsystem IL via normal incidence reflectors 13, 14 onto a reticle or maskpositioned on reticle or mask table MT. A patterned beam 17 is formedwhich is projected by projection system PL via reflective elements 18,19 onto wafer stage or substrate table WT. More elements than shown maygenerally be present in the illumination system IL and projection systemPL.

One of the reflective elements 19 has in front of it an NA disc 20having an aperture 21 therethrough. The size of the aperture 21determines the angle α_(i) subtended by the radiation beam 17 as itstrikes the substrate table WT.

FIG. 2 shows the spectral purity filter 11 according to the presentinvention positioned downstream of the collector 10 and upstream of thevirtual source point 12. In alternative embodiments to that shown inFIG. 2, if the spectral purity filter 11 according to the presentinvention is mainly to be used for mitigating debris emitted fromradiation source LA, then the spectral purity filter 11 is placedbetween the gas barrier or foil trap 9 and the collector 10. In otherembodiments, where the spectral purity filter 11 is mainly to be usedfor spectral filtering, then the spectral purity filter 11 may be placedat any of the following positions: between the collector 10 and thevirtual source point 12 (i.e. the intermediate focus); between thevirtual source point 12 and incidence reflector 13; between incidencereflector 13 and incidence reflector 14; between incidence reflector 14and the mask table MT; and above the substrate table WT.

FIG. 3 depicts a spectral purity filter 100 according to an embodimentof the present invention. Spectral purity filter 100 has a multi-layeredstructure formed by 50 alternating Zr/Si layers 102. Alternativeembodiments may have between 2-200 alternating Zr/Si layers 102.

The spectral purity filter 100 also includes a mesh 104. The mesh 104 ismade of Cu and forms a honeycomb structure including substantiallyhexagonal shaped apertures with a size of about 1-1.5 mm². The mesh 104penetrates from one side to the other side of the alternating Zr/Silayers 102. In alternative embodiments, meshes 104 may be placedadjacent to one side only or on both sides of the Zr/Si layers 102, ormay partially penetrate into the Zr/Si layers 102.

The mesh 104 enhances the integral strength of the Zr/Si layers 102.

The Zr/Si layers 102 are mounted in a substantially annular shaped base106. The shape of the annular shaped base 106 facilitates theincorporation of the spectral purity filter 100 into a lithographicapparatus. Spectral purity filter 100 is therefore easy to handle.

The Zr/Si layers 102 are designed to be substantially robust. Forexample, Zr/Si layers 102 as shown in FIG. 3 with a mesh and with atotal thickness of about 200 nm and a surface area of 1 cm² canwithstand pressure differences up to 0.5-1 bar.

FIG. 4 shows a cross-section of part of the spectral purity filter 100shown in FIG. 3. In FIG. 4, the thickness of the Zr layers 108 is about1 nm and the thickness of the Si layers 110 is about 3 nm. FIG. 4 showsthe mesh 104 extending through the Zr/Si layers 102. In alternativeembodiments, although not shown, the thicknesses of the Zr/Si layers 102may be variable. Although not fully shown in FIG. 4 there may be 50alternating layers of Zr and Si.

FIG. 5 depicts a spectral purity filter 200 according to an embodimentof the present invention. A multi-layer structure formed by alternatingZr/Si layers 202 mounted in a substantially annular shaped base 206. Incontrast to the spectral purity filter 100 shown in FIGS. 3 and 4 thereis no mesh. As there is no mesh, the Zr/Si layers 202 are not as strongas the Zr/Si layers 102. For example, Zr/Si layers 202 with a totalthickness of 200 nm and a surface area of 1 cm² can withstand pressuredifferences of only about 0.1 m bar.

FIG. 6 is a cross-section of part of spectral purity filter 200 shown inFIG. 5. In FIG. 6, the thickness of the Zr layers 208 is about 1 nm andthe thickness of the Si layers 210 is about 3 nm. In alternativeembodiments, although not shown, the thickness of the Zr/Si layers 202may be variable. Although not fully shown on FIG. 6, there may be 50alternating layers of Zr and Si.

Unlike prior art spectral purity filters, the spectral purity filters100, 200 are easily mountable in a lithographic apparatus and may alsobe easily removed. Additionally, although not shown, the spectral purityfilters 100, 200 may be made in a modular form and may therefore formany required surface area for a spectral purity filter.

Using the spectral purity filters 100, 200 shown in FIGS. 3 to 6,effective filtering of DUV is obtainable. The spectral purity filters100, 200 usually have only about a 20% light loss with up to about a100×10⁵ gain in EUV to DUV ratio.

In addition, the spectral purity filters 100, 200 according to thepresent invention mitigate debris such as atomic particles,micro-particles and ions emitted produced from a radiation source.

Table 1 below shows a variety of spectral purity filters according tothe present invention. TABLE 1 Structure d (nm) d(1)/d N h (nm) d(mm)ΔP(bar) Zr 65 200 6 0.12 Zr/Si 3.9 0.75 30 120 6 0.12 Zr/Si 3.9 0.75 65255 6 0.42 Zr/Si 3.5 0.85 75 260 6 0.40 Zr/Si 2.0 0.75 130 260 6 0.56Zr/B₄C 4.0 0.75 60 240 6 0.18 Mo/Si 3.7 0.70 70 260 5 0.52 Zr/Si on 3.90.75 65 255 12 0.45 Mesh Cr/Sc 3.2 0.47 200 635 6 0.47 Cr/Sc 3.2 0.47150 480 7 0.17

Table 1 shows a variety of parameters for the spectral purity filters.In Table 1, the parameters referred to are as follows: d(nm) is thethickness of two alternating layers; d(1)/d is the ratio of thethickness of the two alternating layers; N is the number of alternatinglayers; h(nm) is the total thickness of the alternating layers; d(mm) isthe diameter of the spectral purity filter; and ΔP(bar) is the pressuredifference which the spectral purity filter can withstand. It is worthnoting that the Zr/Si on mesh filter has a relatively large diameter of12 mm but is still able to withstand pressure differences of up to 0.45bar. The mesh therefore adds further strength to the filter.

FIG. 7 relates to calculated and measured spectral transmission valuesfor filters according to the present invention. In particular, FIG. 7shows the high DUV-UV-IR suppression by filters according to the presentinvention. FIG. 7 represents absolute transmission (T) versus wavelength(λ) of radiation. The plotted points are actual values with the curvesbeing calculated. Each of the spectral purity filters has a structureincluding a mesh as shown in FIGS. 3 and 4 and has a total thickness ofabout 200 nm. For the Nb/Si filter, the Nb has a thickness of about 3-4nm and the Si has a thickness of about 0.5-1 nm. For the Mo/Si filter,the Mo has a thickness of about 3-4 nm and the Si has a thickness ofabout 0.5-1 nm. For the Zr/Si filter, the Zr has a thickness of about3-4 nm and the Si has a thickness of about 0.5-1 nm. For the Mo/Cfilter, the Mo has a thickness of about 3-4 nm and the C has a thicknessof about 0.5-1 nm.

To examine the performance and reliability of spectral purity filtersaccording to the present invention a number of experiments wereperformed. These are discussed below.

A. Cold Experiment

The apparatus 300 shown in FIG. 8 was used in a cold experiment. Theapparatus 300 includes a source 302, a foil trap (FT) 304, a collector306 and a Zr/Si spectral purity filter 308 according to the presentinvention. The source 302 is a Xe source and is used to check thespectral purity filter 308 for resistance to high heat and EUV loads.

The experiment was performed while the spectral purity filter 308remained relatively cold due to good conductive cooling of a mount (notshown). The spectral purity filter 308 was placed in an intermediatefocus of the collector 306.

The spectral purity filter 308 is a Zr/Si filter as shown in FIGS. 3 and4 with 50 alternating layers of Zr and Si and a total thickness of 200nm. A mesh also penetrates from one side of the Zr/Si alternating layersto the other.

The experimental conditions are shown below in Table 2. TABLE 2 Numberof shots 5.5 M shot Flux 2 W/cm² Repetition Rate 600 Hz Spot Size 12 mm

The result of the experiment was that no additional damage was observedon the spectral purity filter 308 after 5.5 M shots.

B. Hot Experiment

FIG. 9 relates to apparatus 400 for conducting a hot experiment. Theapparatus 400 includes a Xe source 402, a FT 404 and a Zr/Si spectralpurity filter 408 as used in the cold experiment.

As shown in FIG. 9, the spectral purity filter 408 is mounted asthermally isolated as possible [1 cm×1 mm pin] in order to reach as higha temperature as possible.

The temperature of the spectral purity filter 408 was measured with athermocouple pyrometer.

Table 3 shown below shows three tests performed using the hotexperiment. TABLE 3 Test 1 Test 2 Test 3 Number of shots (M shot) 1.441.3 1.1 Flux (W/cm²) 1.1 2.2 3.5 Repetition Rate of Source (Hz) 300 600954 Spot Size (mm) 12 12 12 Mount/filter temperature, max (° C.) 187 270340

It was found that the spectral purity filter 408 according to thepresent invention withstood the conditions of the experiment well. Someholes in the spectral purity filter 408 which had already existed,developed into a hole the size of the cell in the honeycomb structure ofthe mesh which had aperture sizes of about 1-1.5 mm².

FIG. 10 a shows a surface of the spectral purity filter 408 not yetexposed to radiation in the hot experiments. In contrast, FIGS. 10 b-10d show a number of representations of the spectral purity filter 408once exposed to the hot experimental conditions. FIGS. 10 c and 10 d areenlarged views of FIG. 10 b. It can be observed from FIGS. 10 b-10 dthat although holes 410 may be formed in the spectral purity filter 408of about the same size of the cells in the honeycomb structure of themesh i.e. 1 mm², larger holes did not form. This shows that the mesh inthe spectral purity filter 408 supported and strengthened the Zr/Silayers.

It should be noted that the cold and hot experiments conducted above,were performed with a Xe source, meaning that the conditions are moreextreme than expected in a standard EUV lithographic apparatus.Moreover, the power per shot is about 2.5 times higher than a standardshot in a lithographic apparatus and thus the momentary heating in apulse is also much higher than conditions generally used in alithographic apparatus.

The spectral purity filters as described above may be used in anysuitable type of lithographic apparatus. Moreover, the spectral purityfilters according to the present invention may be used in combinationwith at least one grazing incidence mirror in a lithographic apparatus.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. It should be appreciated that, in the context of suchalternative applications, any use of the term “wafer” or “die” hereinmay be considered as synonymous with the more general terms “substrate”or “target portion”, respectively. The substrate referred to herein maybe processed, before or after exposure, in for example a track (a toolthat typically applies a layer of resist to a substrate and develops theexposed resist), a metrology tool and/or an inspection tool. Whereapplicable, the disclosure herein may be applied to such and othersubstrate processing tools. Further, the substrate may be processed morethan once, for example in order to create a multi-layer IC, so that theterm substrate used herein may also refer to a substrate that alreadycontains multiple processed layers.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the present invention as described without departing fromthe scope of the claims set out below.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the present invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm),X-ray and extreme ultra-violet (EUV) radiation (e.g. having a wavelengthin the range of 5-20 nm), as well as particle beams, such as ion beamsor electron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the present invention have been describedabove, it will be appreciated that the present invention may bepracticed otherwise than as described. For example, the invention maytake the form of a computer program containing one or more sequences ofmachine-readable instructions describing a method as disclosed above, ora data storage medium (e.g. semiconductor memory, magnetic or opticaldisk) having such a computer program stored therein.

1. A lithographic spectral purity filter comprising a multi-layeredstructure of alternating layers, wherein the spectral purity filter isconfigured to enhance the spectral purity of a radiation beam byreflecting or absorbing undesired radiation, the spectral purity filteralso being configured to collect debris emitted from a radiation source.2. A lithographic spectral purity filter according to claim 1, whereinthe spectral purity filter is configured to reflect or absorb DUVradiation while transmitting EUV radiation.
 3. A lithographic spectralpurity filter according to claim 1, wherein at least 90% of EUVradiation in a radiation beam is capable of being transmitted throughthe spectral purity filter.
 4. A lithographic spectral purity filteraccording to claim 1, wherein on transmission of a radiation beamthrough the spectral purity filter, the ratio of EUV radiation to DUVradiation is enhanced by up to 10⁵ times.
 5. A lithographic spectralpurity filter according to claim 1, wherein there is between 2 to 200alternating layers forming the multi-layered structure.
 6. Alithographic spectral purity filter according to claim 1, wherein thereis between 20 to 50 alternating layers forming the multi-layeredstructure.
 7. A lithographic spectral purity filter according to claim1, wherein the thickness of each of the alternating layers forming themulti-layered structure ranges from about 0.5 to 20 nm.
 8. Alithographic spectral purity filter according to claim 1, wherein thetotal thickness of the multi-layered structure of alternating layersranges from about 10 to 700 nm.
 9. A lithographic spectral purity filteraccording to claim 1, wherein the alternating layers forming themulti-layered structure are formed from a combination of any of thefollowing: Zr and Si layers; Zr and B₄C layers; Mo and Si layers; Cr andSc layers; Mo and C layers; and Nb and Si layers.
 10. A lithographicspectral purity filter according to claim 1, wherein the multi-layeredstructure of alternating layers has a mesh-like structure embeddedtherein.
 11. A lithographic spectral purity filter according to claim10, wherein the mesh-like structure is in the form of a honeycomb with aplurality of apertures with a size of about 1 mm².
 12. A lithographicspectral purity filter according to claim 1, wherein the multi-layeredstructure of alternating layers is supported on one side by a mesh-likestructure.
 13. A lithographic spectral purity filter according to claim12, wherein the mesh-like structure is in the form of a honeycomb with aplurality of apertures with a size of about 1 mm².
 14. A lithographicspectral purity filter according to claim 13, wherein the multi-layeredstructure of alternating layers is supported on both sides by amesh-like structure.
 15. A lithographic spectral purity filter accordingto claims 10, wherein the mesh-like structure is in the form of ahoneycomb with a plurality of apertures with a size of about 1 mm². 16.A lithographic spectral purity filter according to claim 1, whereindebris capable of being collected from a radiation source is selectedfrom any combination of the following: atomic particles, micro-particlesand ions.
 17. A lithographic apparatus, comprising: an illuminationsystem configured to condition a radiation beam; a support configured tosupport a patterning device, the patterning device being configured toimpart the radiation beam with a pattern in its cross-section to form apatterned radiation beam; a substrate table configured to hold asubstrate; a projection system configured to project the patternedradiation beam onto a target portion of the substrate; and a spectralpurity filter comprising a multi-layered structure of alternating layersand configured to enhance the spectral purity of the radiation beam byreflecting or absorbing undesired radiation, the spectral purity filteralso being configured to collect debris emitted from a radiation source.18. A lithographic apparatus according to claim 17, wherein the spectralpurity filter is positioned in a source-collector-module of thelithographic apparatus.
 19. A lithographic apparatus according to claim17, wherein the spectral purity filter is positioned in the illuminationsystem of the lithographic apparatus.
 20. A lithographic apparatusaccording to claim 15, wherein the spectral purity filter is positioneddownstream of a collector and upstream of an intermediate focus of theradiation beam.
 21. A lithographic apparatus comprising a spectralpurity filter including a multi-layered structure of alternating layers,wherein the spectral purity filter is configured to enhance the spectralpurity of a radiation beam by reflecting or absorbing undesiredradiation, the spectral purity filter also being configured to collectdebris emitted from a radiation source.
 22. A device manufacturingmethod, comprising: providing a beam of radiation; patterning the beamof radiation; projecting a patterned beam of radiation onto a targetportion of the substrate; and enhancing the spectral purity of the beamor radiation by reflecting or absorbing undesired radiation using aspectral purity filter comprising a multi-layered structure ofalternating layers.
 23. A device manufacturing method according to claim22, further comprising: collecting debris emitted from a radiationsource with the spectral purity filter
 24. A device manufacturedaccording to the method of claim
 22. 25. A device according to claim 24,wherein the device is an integrated circuit; an integrated opticalsystem; a guidance and detection pattern for magnetic domain memories; aliquid crystal display; or a thin-film magnetic head.